Rework Roo::Base#initialize so that Roo::CSV can use it to initialize.

Previously none of the classes were. :-P
Make the rest of our classes be initialized in part by Base.
Make all the classes consistent in that #read_cells is only effective if the sheet has not been read.
Roo's roo_rails_helper, aka the `spreadsheet` html-generating view method is currently deprecated with no replacement.

It is a mess with no clear general use-case. People should write their own views for this stuff if necessary.